Allow secret generators to access order datetime (#4110)

src/pretix/base/secrets.py

@@ -257,6 +257,8 @@ def assign_ticket_secret(event, position, force_invalidate_if_revokation_list_us
         kwargs['valid_from'] = position.valid_from
     if 'valid_until' in params:
         kwargs['valid_until'] = position.valid_until
+    if 'order_datetime' in params:
+        kwargs['order_datetime'] = position.order.datetime
     secret = gen.generate_secret(
         item=position.item,
         variation=position.variation,